Solving the Azobenzene Entropy Puzzle: Direct Evidence for Multi-State Reactivity

Area of Science:

• Chemical Physics
• Physical Chemistry
• Photochemistry

Background:

• Azobenzene thermal Z→E isomerization exhibits large negative activation entropies, unexplained by previous computational models.
• The "entropy puzzle" refers to the discrepancy between experimental observations and theoretical predictions for azobenzene isomerization.

Purpose of the Study:

• To resolve the azobenzene "entropy puzzle" by proposing and validating a new isomerization mechanism.
• To explain the experimentally observed large negative activation entropies in azobenzene thermal isomerization.

Main Methods:

• Utilized nonadiabatic transition state theory for computational analysis.
• Conducted new experiments to probe the reaction mechanism, including investigating the heavy-atom effect.

Main Results:

• The thermal Z→E isomerization of azobenzene proceeds via a complex multistate rotation mechanism involving a triplet excited state.
• Calculated Eyring parameters show excellent agreement with experimental data, validating the proposed mechanism.
• Experimental evidence for a significant external heavy-atom effect supports the importance of spin-orbit coupling.

Conclusions:

• The azobenzene "entropy puzzle" is solved by a mechanism incorporating triplet excited states and spin-orbit coupling.
• The findings necessitate a reevaluation of mechanisms for similar thermal double bond isomerizations where excited states become accessible.